1. Field of the Invention
The present invention relates to a surface modified quartz glass crucible and its modification process, in which the crucible is used in the process of pulling up silicon single crystal used for a semiconductor, etc., from molten silicon.
2. Discussion of the Background
As one of the causes that a dislocation is formed in a silicon single crystal during pulling up of the crystal, it has been known that cristobalite formed on the inside surface of the quartz glass crucible is released into the molten silicon. It is also known that a process in which an alkaline earth metal is coated on the inside surface of the crucible as a crystallization accelerator to form cristobalite layer on the inside surface of the crucible at an early sage of the pulling up (e.g. U.S. Pat. No. 5,976,247 or Japanese Pat. No. 3100836), is a counter measure to this problem. In these inventions, a barium hydroxide solution etc. is coated on the surface of the quartz glass crucible and the coated barium hydroxide reacts with carbon dioxide in air to form barium carbonate on the surface of the crucible. Then, the formed barium carbonate is weakly adhered on the surface of the crucible by drying it and is used as the crystallization accelerator.
However, the alkaline earth metals beryllium, magnesium, calcium, and strontium, have a large segregation coefficient, so that there is a fatal problem that these metals can be easily included in the single crystal silicon to significantly reduce its semiconductor properties. Moreover, although barium has a small segregation coefficient, there is also a problem that handling is difficult since its toxicity is high. Therefore, a quartz glass crucible having a surface layer which contains elements selected from zirconium, niobium, hafnium, tantalum, and rare earth elements, is also proposed (Japanese Patent Laid Open No. 2002-29890) instead of the alkaline earth metal.
However, there are the following problems in the above-described strategy.
That is, regarding the former quartz glass crucible, (A) since the barium carbonate powder on the surface of the crucible is only weakly adhered without any binder etc., the adhesion strength of the powder is so weak that the barium carbonate powder is abraded easily to a non-uniform adhesion state. Moreover there is also a possibility that the worker's health its injured because the barium carbonate powder is scattered. In addition, (B) the adhesion amount of barium carbonate becomes excessive easily. Furthermore, (C) if the crucible is washed, the barium carbonate powder adhered on the surface of the crucible is washed away. Therefore it is impossible to wash the crucible after the adhesion of the barium carbonate powder, even if some soils adhere on the surface of the crucible.
Moreover, regarding the latter quartz glass crucible, the surface modified layer is formed in a process wherein a coating liquid comprising a metal oxide dispersed in a sol state is coated on the surface of the crucible and dried at 150° C. in the atmosphere. However, the surface modified layer baked on the surface of the crucible cannot be formed by heat-treatment at such low temperature, so that the problem, i.e., the surface layer is abraded easily, cannot be solved.